1Unit 4 - 54/11/2017E2 ReactionsE2 = elimination, bimolecularReaction is concerted (one step)Rate = k[substrate][base] (a second order process)E2 can occur with SN2.Occurs by abstraction of H+ from a C adjacent to the C with the LG.Products follow Zaitsev’s Rule.If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.
2E2 Reaction Mechanism One-step and requires a strong base. Best transition state is anti-coplanar.Example: t-butyl bromide + methoxideWhy don’t SN2, SN1, or E1 occur?
6Factors Affecting E2 Reactions Structure of the substrateStrength of the baseNature of the leaving groupThe solvent in which the reaction is run.
7Factors Affecting E2 Reactions - Structure of the Substrate Structure of the alkyl halide: 3° > 2° >1°This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).
8Factors Affecting E2 Reactions - Structure of the Substrate Decide whether the following substrates could react by E1 or E2 (and by SN1 or SN2).
9Factors Affecting E2 Reactions - Strength of the Base The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?
10Factors Affecting E2 Reactions - the Leaving Group The LG should be good. Which of the following substrates have good LGs?
11Factors Affecting E2 Reactions - Solvent Effects Polarity is not so important because negative charge is spread over the transition state.
12Zaitsev’s RuleWhen two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.
13Zaitsev’s Rule Show the E2 elimination products that could come from: Which E2 reaction would occur more quickly?
14E2 Reactions - SummaryThe structure of the substrate affects the rate.Relative rates for E2: 3°>2°>1°.The base must be strong.The LG should be good.The solvent should be polar.Coplanar (usually anti) transition state is required.Products will follow Zaitsev’s Rule.Can occur with SN2 for 2° alkyl halides.
17polar, aprotic is better Unit 4 - 54/11/2017E1 and E2 ReactionsE1E2Baseweak ones workstrongSubstrate3°>2°3°>2°>1°Leaving groupgoodSolventpolar, ionizingpolar, aprotic is better
18E1 and E2 Reactions E1 E2 Rate =k[RX] =k[RX][Base] Unit 4 - 54/11/2017E1 and E2 ReactionsE1E2Rate=k[RX]=k[RX][Base]Carbocation intermediate?YNStereochemistryZaitsev’s ruleRearrangement~H, ~ CH3 possibleNo rearrangements
19How Do You Decide Which Reaction(s) Happen? The strength and structure of the nucleophile/base is paramount.Strong ones always give a second order reaction (SN2 or E2).If the base is bulky, E2 will occur instead of SN2.
20How Do You Decide Which Reaction(s) Happen? Now, look at the structure of the substrate.1° alkyl halides will undergo SN2 and perhaps E2 unless rearrangement is possible.3° alkyl halides will NOT undergo SN2.2° alkyl halides are the toughest to predict.
21How Do You Decide Which Reaction(s) Happen? Evaluate, in order,Nucleophile, strength and structure-or- Base, strength and structureStructure of the alkyl halide/carbocationLeaving groupSolventYOU MUST BE ABLE TO DRAW THE MECHANISM!